
theorem :: AChain36a:
for R being non empty RelStr, a1, a2 being Element of R
  st a1 <> a2 & {a1,a2} is StableSet of R holds not (a1 <= a2 or a2 <= a1)
proof
 let R be non empty RelStr, a1,a2 be Element of R;
 assume A1: a1 <> a2;
A2: a1 in {a1,a2} & a2 in {a1,a2} by TARSKI:def 2;
  assume {a1,a2} is StableSet of R;
  hence thesis by A2,A1,Def2;
end;
